1. Field of the Invention
The present invention relates to an image forming system.
2. Description of the Related Art
Conventionally, there is known a tandem-type image forming system to improve the productivity, the tandem-type image forming system in which image forming apparatuses such as a printer and a copier which form images on sheets of paper are disposed in series, whereby double-sided printing is performed by using two image forming apparatuses which respectively form images on the sides of each sheet of paper.
For example, Japanese Patent Application Laid-Open Publication No. hei 8-234605 discloses a tandem-type image forming system including a first electrophotographic apparatus (provided on an upstream side) which forms an image on the front side of a sheet of paper, and a second electrophotographic apparatus (provided on a downstream side) which forms an image on the back side of the sheet thereof, wherein the fixing temperature of the second electrophotographic apparatus is appropriately controlled. More specifically, there is a plurality of heaters inside a heat roller of a fixing device of each of the first and second electrophotographic apparatuses, and the heaters heat different parts of the heat roller, for example, the center part of the heat roller, the end part thereof, and the whole thereof. In this tandem-type image forming system, when double-sided printing is performed, control is performed in order that the number of the heaters of the heat roller of the fixing device of the second electrophotographic apparatus, the heaters which heat a boundary part between a paper travelling area and a paper non-travelling area, becomes less than the number of the heaters of the heat roller of the fixing device of the first electrophotographic apparatus, whereby the temperature at the boundary part is prevented from going up too much.
In such a conventional tandem-type image forming system, when printing is continuously performed, the temperature inside the second electrophotographic apparatus (inside temperature, hereinbelow) goes up. When the inside temperature goes up too much, it is possible that toners in a develop device thereof melt. This is because a paper path from the fixing device of the first electrophotographic apparatus to a paper receiving portion of the second electrophotographic apparatus is short, and because the second electrophotographic apparatus receives a sheet in a short time after the first electrophotographic apparatus fixes an image on the sheet.
It is also possible that an image formed on a sheet becomes grainy. This is because, in the second electrophotographic apparatus, not only the inside temperature, but also the temperature of the fixing device goes up.